Distillation of pentachlorophenol with salicylaldehyde and water

ABSTRACT

Purified pentachlorophenol is recovered in good yield by distilling the commercially available grade in the presence of salicylaldehyde and water under reduced pressure. The purified product is nearly colorless and nonblooming and it is essentially free of polychlorinated dibenzo-p-dioxins and phenoxyphenols.

United States Patent 11 1 1111 3,852,160

Watson et al. 1 Dec. 3,1974

[ DISTILLATION 0F 3,769,353 10/1973 Francis 6161. 260/623 11 PENTACHLOROPHENOL WITH 3,770,835 11/1973 Garabedian 260/623 R SAUCYLALDEHYDE AND WATER FOREIGN PATENTS OR APPLICATlONS Inventors: am avi Watson; Erwin R 1,246,529 9/1971 Great Britain 260/623 R Kohel, both of Midland, Mich. I OTHER PUBLICATIONS [73] AssIgnee: TheDow ChemIcal Company, 7

Midland, Mich. BIltz et al., BerIchte 37, 4017,4018 (1904).

F' 1 2 [22] 3 1973 Primary Exammer-Wflbur L. Bascomb, Jr. [21] Appl. No.: 363,224 Attorney, Agent, or Firm-David T. Thurston [52] US. Cl 203/6, 203/38, 203/92, [57 ABSTRACT 203/54, 203/62, 260/623 R, 260/623 1-1 51 Int. Cl B01d 3/34, C070 39/36 Punfied Pentachlorophenol 1S recovered In good w 58 Field Of Search 203/54, 6, 62 96,91; by distilling the commercially available grade in the 260/623 623 H presence of salicylaldehyde and water under reduced pressure. The purified product is nearly colorless and [56] References Cited nonblooming and it is essentially free of poly- UNITED STATES PATENTS chlorinated dibenzo-p-dioxins and phenoxyphenols.

2,662,918 12/1953 Spaulding 260/623 H 3 Claims, No Drawings DISTILLATION OF PENTACHLOROPHENOL WITH SALICYLALDEHYDE AND WATER BACKGROUND OF THE INVENTION This invention relates to an improved method whereby pentachlorophenol can be purified by distilla tion with high recovery and minimal decomposition and side reactions. A product which is essentially free of undesirable impurities and unsatisfactory physical properties is thereby obtained.

Pentachlorophenol is a well known and widely used fungicide and preservative which is particularly useful for preserving wood exposed to attack by the elements. Commercially available pentachlorophenol in the past has been a relatively dark colored solid sold in the form of flakes and prills or beads. This material had an annoying characteristic of blooming or subliming to form a surface layer of easily dislodged fine crystalline dust which is intensely irritating to skin and mucous membranes. The dark color is caused by high molecular weight tarry impurities and is of itself no particular disadvantage for many uses such as the protection from rot of posts and poles designed for sinking in the ground. However, the color is disadvantageous for other uses, for example wood used in outdoor fumiture, porches, and decks. Of perhaps more serious consequence is the recent discovery of the toxic properties of polychlorinated dibenzo-p-dioxins which fortunately constitute a relatively small proportion of the high mo- LII lecular weight impurities. It has also been found that other polychlorinated polynuclear impurities, the chlorinated phenoxyphenols, are a primary cause of blooming of impure pentachlorophenol.

Impurities such as these can be removed by recrystallization, but this process is usually too expensive and slow for use with a bulk chemical. An obvious means for getting rid of high boiling impurities is distillation and pentachlorophenol has been distilled to purify it, see Biltz, et al., Berichte 37, 4018 (1904). However, pentachlorophenol is not completely stable at elevated temperatures and it tends to decompose and undergo side reactions to some extent during distillation to form large volumes of HCl and tar, particularly in the presence of the small amounts of aluminum and iron chlorides present in most commercially available material.

Exposure of the material to water deactivates these metal chlorides but an undesirable degree of decomposition and side reaction persists at distillation temperatures. It is known that small amounts of salicylaldehyde stabilize pentachlorophenol at elevated temperatures, see British Pat. No. 1,246,529, but even in its presence, an undesirable amount of decomposition continues in the prolonged heating necessary for distillation.

SUMMARY OF THE INVENTION DETAILED DESCRIPTION It is known, as previously mentioned, that salicylaldehyde inhibits decomposition of molten pentachlorophenol and it might be expected to have a similar effect when the pentachlorophenol is distilled. It is also com- 4 mon practice, of course, to deactivate traces of ferric chloride or aluminum chloride by adding water to F riedel-Crafts reaction mixtures before distilling the products. However, the stabilizing effect found when technical grade pentachlorophenol is distilled in the presence of the claimed combination is considerably greater than any additive effect which might be predicted from the known and demonstrated effects of salicylaldehyde and water used separately. The claimed invention, therefore, is based on surprising synergistic stabilizing effect exerted by the two components when used in combination.

The water component can be added in any of several convenient ways, either before or during the distillation. Liquid water may be added or the pentachlorophenol may be contacted with steam. Even contact of the pentachlorophenol with moist air is effective.

A suitable proportion of salicylaldehyde is about 0.05-2 percent by weight and preferably about 0.l-l percent. These figures represent practical limits rather than critical amounts, for any significant amount provides some stabilizing effect. Larger amounts actually begin to show a deleterious effect. At least about 0.01 percent of water is preferred and about 0.05-0.5 percent is most preferred. Larger quantities can be used but give little or no added benefit. A

For reasons of convenience and avoidance of significant decomposition even in the presence of the stabilizing combination, the distillation is carried out at subatmospheric pressure, preferably below 100 mm. Hg absolute, and most preferably at an absolute pressure sufficiently low to keep the distillation head temperature below about 230C, i.e., a pressure below about 80 mm. Hg, but sufficiently high to avoid freezing of the pentachlorophenol in the head.

The distilled product is typically a light yellow crystalline solid having little or no tendency to bloom. When it is converted to a granular form such as flakes or beads, it remains free flowing and essentially unchanged in appearance even after long storage.

Examples 1-2 Samples of 500 g. technical grade pentachlorophenol prills with or without additive as noted were distilled at mm. Hg absolute pressure through'a distillation column packed with stainless steel column packing in about 1 hour of distillation time to a final pot temperature of about 270C. except as noted. The pentachlorophenol had been made by chlorination of chlorophenols in the presence of aluminum chloride. The residue remaining in the distillation pot was then held at the final pot temperature for an additional four hours to simulate recycling of tars through a heat exchanger in a commercial distillation unit. The I-ICl evolved during the distillation and during the hold period was monitored by scrubbing the vent gas through a known quantity of 1N NaOH and titrating with 1N HCl. The results are listed in the table along with those of various control experiments with or without additives as noted which were run for purpose of comparison. The products obtained by distillation in the presence of the 3 4 salicylaldehyde-water combination were typically tively high, comparable to that obtained in the control about 90-95 percent pentachlorophenol with the reexperiments of Table 1 when there was no additive or mainder consisting essentially of tetrachlorophenol. when water alone or salicylaldehyde alone was used. Only trace amounts if any of polychlorinated phenoxy' This indicates a continuing, unacceptably high rate of phenol and polychloro dibenzo-p-dioxin were present. 5 decomposition during the long heating periods re- TABLE 1 Millimoles HCl Evolved Wt. Recovery During Example No. Additive Distillate Residue Distillation At 270-300C.

1 1.9 g. sulicylaldehydc 95.0 4.9 1.1 3.0

water 2 1.9 g. salicylaldehydc 92.4 7.4 0.2 1.7

+ 0.37 g. water A none 84.5 13.8 113.7 40.7 B water 93.3 6.0 11.6 34.2 C 1.9 g. salicylaldehydc 89.2 9.9 25.6 34.3 D 3.8g. salicylaldehydc 84.9 13.8 122.8 55.8

water was added by blowing moist air through the prills for -15 minutes.

Surprisingly, thehighl y efiective inhibition of decomqu ired in a plant distillation procedure. position during distillation shown by the salicylalde- We clai hyde-water combination appears to be unique to that 1, A process for purifying impure pentachlorophenol combination and the effect is not shown to that extent hi h comprises di tilli aid pentachlorophenol at y mixtures of Water with Similar aromatic aldehydessubatmospheric pressure in the presence of contacting Table 2 lists the results obtained when the procedure water d b t 005-2 pergent f ii l ld h d of Example 2 is repeated U g aromatic aldehydes based on the weight of pentachlorophenol, thereby sep- 0 other lhan salwylaldehydearating purified pentachlorophenol as a distillate from TABLE 2 Millimoies HCl Evolved Wt. Recovery During Additive Distillate Residue Distillation At 270300C.

1.9 g. p-hydroxy-benzaldehyde 93.2 6.6 1.5 21.9

0.37 g. 11,0 1.9 g. benzaldehyde 92.0 7.7 3.8 19.6

0.37 g. H O 1.9 g. O-anisaldehyde 92.6 7.2 2.5 29.4

0.37 g. 11,0 1.9 g. S-methyl-salicylaldehyde 92.6 7.3 0.6 52.3

lt viill be notediiiih' Eifiifiiiflii'ifiidiiiidti h higher boiling impurities? WW 3 the combinations of water with the other aromatic al- 2. The process of claim 1 wherein about 0.1-1 perdehydes gave reasonably good results in the recovery cent of salicylaldehyde and at least about 0.01 percent of distilled pentachlorophenol, in tar production, and of water are present.

in the HCl evolved during distillation, the HCl evolved" 3. The process of claim 2 wherein the distillation from the tar residue during the holding period was rela-. pressure is below 100 mm. Hg absolute.

. it It i 1' 

1. A PROCESS FOR PURIFYING IMPURE PENTACHLOROPHENOL WHICH COMPRISES DISTILLING SAID PENTACHLOROPHENOL AT SUBATMOSPHERIC PRESSURE IN THE PRESENCE OF CONTACTING WATER AND ABOUT 0.05-2 PERCENT OF SALICYLALEHYDE BASED ON THE WEIGHT OF PENTACHLOTROPHENOL, THEREBY SEPARATING PURIFIED PENTACHLOROPHENOL AS A DISTILLATE FROM HIGHER BOILING IMPUTRITIES.
 2. The process of claim 1 wherein about 0.1-1 percent of salicylaldehyde and at least about 0.01 percent of water are present.
 3. The process of claim 2 wherein the distillation pressure is below 100 mm. Hg absolute. 